At the present time, as the bonding wire for bonding an electrode on a semiconductor device and an external terminal, a thin wire of a wire size of 20 to 50 μm or so (bonding wire) is mainly being used. For bonding a bonding wire, the ultrasonic bonding/thermal compression bonding method is the most general. A general purpose bonding machine and a capillary jig through which the wire is passed for connection are used. The tip of the wire is heated to melt by arc heat input, the surface tension is used to form a ball, then this ball part is pressed against an electrode of the semiconductor device heated to 150 to 300° C. in range for bonding, then the wire is directly bonded to the external lead side by ultrasonic bonding.
In recent years, the structures, materials, connection techniques, etc. for mounting semiconductors have rapidly diversified. For example, in the mounting structures, in addition to the current QFP (Quad Flat Packaging) using lead frames, the BGA (Ball Grid Array), CSP (Chip Scale Packaging), or other new methods using the substrate, polyamide tape, etc. have come into use. Bonding wire improved more in loop property, bondability, use for mass production, etc. is being sought. In the connection techniques for such wire as well, in addition to the current mainstream ball/wedge bonding, in wedge/wedge bonding suitable for narrower pitches, the wire is directly bonded at two locations, so the improvement of the bondability of the wire is being sought.
The materials to which the bonding wire is bonded are also becoming more diversified. In addition to the conventional Al alloys used for the interconnects and electrodes on silicon substrates, Cu suitable for thin interconnects is being used. Further, lead frames are being plated with Ag, plated with Pd, etc. Further, resin substrates, tapes, etc. are being laid with Cu interconnects which are in turn covered with films of gold or other precious metal elements and their alloys in many cases. In order to handle these various members to be bonded with, improvement of the wire bondability and the bond reliability is being sought.
As the material of the bonding wire, up until now high purity “4N” (purity>99.99 mass %) gold has mainly been used. However, gold is expensive, so bonding wire of other types of metal less expensive in cost of material is desired.
In terms of requirements from the viewpoint of wire bonding techniques, it is required that a ball with good sphericity be formed at the time of ball formation and that the bond between that ball part and the electrode give sufficient bonding strength. Further, in order to handle the increasingly lower bonding temperatures and increasingly finer wires, bonding strength, tensile strength, etc. are also necessary at the location where the wire is wedge connected to an interconnect on a circuit board.
In the resin sealing process where a high viscosity heat curing epoxy resin is injected at a high speed, there is the problem of the wire deforming and contacting the adjacent wires. Further, as pitches become narrower, wires become longer, and wires become thinner, as much reduction of wire deformation as possible at the time of resin sealing is being sought. Increase of the wire strength enables such deformation to be controlled to a certain extent, but loop control becomes difficult, the strength at the time of bonding drops, and other problems arise which have to be solved or else practical application will be difficult.
As wire characteristics for satisfying these requirements, easy loop control in the bonding process and, further, improved bondability to the electrode parts and lead parts, suppression of excessive wire deformation in the resin sealing process after bonding, and other general characteristics are desirably satisfied.
Bonding wire inexpensive in cost of material, superior in electrical conductivity, improved in ball bonding and wedge bonding, etc. by using a copper as a material has been developed and is disclosed in Japanese Patent Publication (A) No. 57-149744 (A), Japanese Patent Publication (A) No. 61-99645, etc. However, with copper bonding wire, there are the problems that oxidation of the wire surface causes the bonding strength to drop and corrosion of the wire surface easily occurs at the time of resin sealing. These are becoming causes preventing practical use of copper binding wires.
Therefore, as a method for preventing surface oxidation of copper bonding wire, Japanese Patent Publication (A) No. 62-97360 proposes wire comprised of copper covered with gold, silver, platinum, palladium, nickel, cobalt, chrome, titanium, or another precious metal or a corrosion resistant metal. Further, from the viewpoint of the ball formability, prevention of degradation of the plating solution, etc. Japanese Patent Publication (A) No. 2004-64033 proposes a wire of a structure of a core material having copper as its main ingredient and a covering layer comprised of a different metal layer comprised of a metal other than copper formed on the core material and an oxidation resistant metal with a melting point higher than copper formed on that different metal layer.
As practical problems in copper bonding wire, the ease of oxidation of the wire surface, the drop in bonding strength, etc. may be mentioned. Further, with high purity copper bonding wire, there are the problems that due to the insufficient wire strength, the wire deformation at the time of resin sealing is large, the pull strength of the neck part is low, low loop formation is difficult, etc., so there is also the problem that there are few semiconductor products to which it can be applied. Therefore, as a means for preventing surface oxidation of copper bonding wire, the wire surface may be covered with a precious metal or oxidation resistant metal.
The inventors evaluated such wire considering the higher mounting density, smaller size, and greater thinness of semiconductor devices and other needs and thereupon learned that many practical problems remain, as explained later, in conventional covered copper wire of a structure of copper bonding wire covered on its surface with a metal different from copper (hereinafter referred to as the “conventional multilayer copper wire”).
When forming a ball at the tip of a conventional multilayer copper wire, there is the problem that a “flat ball” off from a true sphere is formed or unmelted wire will remain inside the ball. If bonding such an abnormal ball part on an electrode, it will cause a drop in bonding strength, chip damage, or other problems. Further, along with the lower loop formation or other tougher loop control, the neck part is easily damaged and the pull strength falls in some cases.
If performing complicated loop control with a conventional multilayer copper wire, detachment at the interface between the covering layer and copper etc. may result in an unstable loop shape or electrical short-circuiting of adjoining wires in narrow pitch connection.
When wedge bonding a conventional multilayer copper wire to an electrode of a circuit board or the like, peeling at the interface between the covering layer and core material, discharge of the covering layer from the bond of the wire and electrode and copper being directly bonded, etc. may cause the bonding strength to become unstable or fall.
As a factor for alleviating these problems in the conventional multilayer copper wire, control of the thickness of the covering layer may be considered. However, if making the covering layer thicker, while an improvement in the wedge bonding etc. may be expected, formation of a thick covering layer by plating, vapor deposition, etc. would cause a drop in productivity, a rise in the costs of materials, and other problems in industrial production. Further, if making the covering layer thicker, there is the problem that the concentration of elements other than copper will rise inside the melted ball and therefore the ball part will end up hardening and chip damage will be given at the ball bond.
As opposed to this, if just making the covering layer of a conventional multilayer copper wire thinner, the problems will arise of peeling at the interface between the covering layer and core material and more difficult prevention of oxidation, improvement of wedge bonding, etc.
Further, to promote the practical use of copper wire in the future, it will be necessary to sufficiently handle thick wire of 50 μm size or more for power IC applications for which gold wire is not used much and thin wire of 20 μm size or less using the high conductivity of copper and, in terms of characteristics, handle the tougher requirements of improvement of the bondability of thick wire, narrow pitch small ball bonding, low temperature bonding, reverse bonding for multilayer chip connection, etc.